Amorphous magneto optical recording medium

ABSTRACT

A magneto optical thin film recording medium is disclosed having very high carrier-to-noise ratios and high rotation angles. A transmission electron microscope photomicrograph (at 200,000 X) of one such medium is shown in FIG. 1. These are multi-phase amorphous materials having magnetic anisotropy perpendicular to the plane of the thin film. They are produced in a triode vacuum sputtering process at vacuums in the range of 4×10 -3  to 6×10 -4  Torr. By adjusting process parameters such as substrate temperature, anode bias and deposition rate, the properties of the thin film can be altered.

This is a continuation of application Ser. No. 874,083, filed June13,1986, now U.S. Pat. No. 4,721,658, which is a continuation ofapplication Ser. No. 599,669, filed Apr. 12,1984, now U.S. Pat. No.4,615,944, which is a continuation-in-part of application Ser. No.495,175, filed May 17, 1983, abandoned.

TECHNICAL FIELD

This invention relates to amorphous thin film magnetic materials. Moreparticularly, it pertains to magnetic compositions having magneticanisotropy, whereby the thin film possesses a stable magnetic easy axisperpendicular to the plane of the film itself. These compositions can beused as light modulators, in which light interacting with the thin filmis affected by the presence of a magnetic domain at the incident spot.

BACKGROUND

Magneto-optic recording media are also known by several other names:thermomagnetic media, beam addressable files, and photo-magneticmemories. All of these terms apply to a storage medium or memory elementwhich responds to radiant energy permitting the use of such energysources as laser beams for both recording and interrogation. Such mediamodify the character of an incident polarized light beam so that themodification can be detected by an electronic device such as aphotodiode.

This modification is usually a manifestation of either the Faradayeffect or the Kerr effect on polarized light. The Faraday effect is therotation of the polarization plane of polarized light which passesthrough certain magnetized media. The Kerr effect is the rotation of theplane of polarization of a light beam when it is reflected at thesurface of certain magnetized media.

Magneto optic recording media have several advantages over knownmagnetic recording media:

1. The spacing between the medium and the recording head is greater,thus reducing potential for contact and wear;

2. Using a pulsed laser beam as the writing means, very high densitydata storage is possible.

3. With an interference layer on top of a magneto optic layer, themedium is affected less by dust than magnetic media.

In magneto optical recording, data is written into a medium having apreferentially directed remanent magnetization by exposing a localizedarea (spot or bit) on the recording medium to an electromagnetic orother energy source of sufficient intensity to heat the recording mediumabove its compensation or Curie point temperature and simultaneouslybiasing the medium with a magnetic field. Preferably, the energy sourceis a laser which produces a monochromatic output beam. The magneticfield required to reverse the magnetization of the recording mediumvaries with the temperature to which the recording medium is brought.Generally speaking for a given material, the higher the temperature, thesmaller the required magnetic field coercive force.

The write or record operation for both Curie point and compensationpoint writing is as follows:

1. The medium is initially in a demagnetized state having about equalnumbers of magnetic domains with magnetization oppositely directed andperpendicular to the surface of the film. A domain will herein refer tothe smallest stable magnetizable region; although, in common usage adomain is a uniformly magnetized region of any size. The medium may besubjected to a saturation magnetic bias field normal to the surface ofthe film in order to magnetize all the domains in one direction.Alternatively, a selected area of the medium may be magnetized byexposing said area to a continous light beam and a small magnetic biasfield.

2. A small magnetic bias field oriented perpendicular to the surface orplane of the film, but oppositely directed to the magnetic field appliedearlier is applied over the entire thin film medium. 3. With the biasingfield in place, a light beam from a radiant energy source such as alaser beam is directed toward a selected location or bit on the filmwhere it causes localized heating of the film to a temperature at orabove the compensation temperature. When the laser beam is removed, thebit cools in the presence of the biasing magnetic field and has itsmagnetization switched to that direction. The medium, in effect, has amagnetic switching field which is temperature dependent. The magneticbiasing field applied to the irradiated bit selectively switches the bitmagnetization, with the bit momentarily near its compensationtemperature under the influence of the laser. The momentary temperaturerise reduces the bit coercive force.

In the write operation, the write laser beam (e.g. about 8-12 mW) isfocused to the desired diameter (e.g. 1.0 microns) onto the surface ofthe recording medium by an objective lens.

The memory element or recorded bit is interrogated, or read,nondestructively by passing a low-power (e.g. 1-3 mW)beam of polarizedlight (e.g. a laser beam) through the bit storage site for asufficiently short time so as not to heat the medium to change itsmagnetic state. The read laser beam is normally shaped to a circularcross-section by a prism, polarized and focused to some small diameter(e.g. 1.0 microns) onto the recording medium by a lens. When the readbeam has passed through the recorded spot, it is sent through an opticalanalyzer, and then a detector such as a photodiode, for detection of anychange or lack of change in the polarization.

A change in orientation of polarization of the light is caused by themagneto-optical properties of the material in the bit or site. Thus, theKerr effect, Faraday effect, or a combination of these two, is used toeffect the change in the plane of light polarization. The plane ofpolarization of the transmitted or reflected light beam is rotatedthrough the characteristic rotation angle θ. For upward bitmagnetization, it rotates θ degrees and for downward magnetization -θdegrees. The recorded data, usually in digital form represented by logicvalues of 1 or O depending in the direction of bit magnetization, aredetected by reading the change in the intensity of light passing throughor reflected from the individual bits, the intensity being responsive tothe quantity of light which is rotated and the rotation angle.

Erasure can be accomplished by simply writing new information over oldportions of the medium or by simply exposing any given bit with a laserbeam of sufficient intensity and then cooling that bit in the presenceof a magnetic field in the direction of the initially applied magneticfield. The entire storage medium can be erased by providing a largemagnetic bias field in the original saturation direction which does notrequire a laser beam. Generally, in the recording process, the externalbiasing magnetic field is applied by a magnet set above or behind themagneto optic medium, and in the erasing process, the magnet is reversedin direction.

The signal-to-noise ratio (SNR) or carrier-to-noise ratio (CNR) of anerasable magneto optic medium is proportional to θ√R, where R equalspower reflectivity of the medium and θ is the angle of rotation.Forty-five decibels in a 30 kHz band width is generally considered theminimum CNR acceptable for direct read after write (DRAW) media. Thespeed at which the bits can be interrogated and the reliability withwhich the data can be read depends upon the magnitude of themagneto-optical properties, such as the angle of rotation, of the thinfilm and upon the ability of the interrogation system to detect theseproperties. An increase in the angle of rotation θ results in anincrease in CNR.

For purposes of this discussion, the noise floor or noise level ismeasured at the average noise level.

The main parameters that characterize a magneto optic material are theangle of rotation, the coercive force (H_(c)) the Curie temperature andthe compensation point temperature. The medium is generally comprised ofa single element or multicomponent system where at least one of thecomponents is an amorphous metal composition. Binary and ternarycompositions are particularly suitable for these amorphous metal alloys.Suitable examples would be rare earth-transition metal (RE-TM)compositions such as: Gadolinium-cobalt (Gd-Co), Gadolinium-iron(Gd-Fe), Terbium-iron (Tb-Fe), Dysprosium-iron (Dy-Fe), Gd-Tb-Fe,Tb-Dy-Fe, Tb-Fe-Co, Terbium-iron-chromium (Tb-Fe-Cr), Gd-Fe-Bi(Bismuth), Gd-Fe-Sn (Tin), Gd-Fe-Co, Gd-Co-Bi, and Gd-Dy-Fe.

Japanese patent publication number 56/143547 discloses a magneto opticmedium of the type just discussed. It comprises a thin film ofgadolinium-terbium-iron alloy in a ratio of 0.24/0.18/1 which film ismore than 1000 angstroms thick when using the Kerr effect and 500 to 800angstroms thick when using the Faraday effect. The film of this patentalso has a 5400 angstrom thick glass (silicon dioxide) film on top ofthe Gd:Tb:Fe film.

The magneto optic amorphous thin films can be fabricated by known thinfilm deposition techniques, such as sputtering, evaporation and splatcooling. In splat cooling a hot liquid of the film constituents isincident on a cool surface where they are quenched and solidifiedrapidly to form an amorphous bulk film. Generally, no matter whatdeposition rate is used, the substrate temperature must be less thanthat at which crystallization occurs in order to provide amorphousmagnetic materials.

The preferred process for thin film deposition is sputtering. Typicalknown sputtering conditions for amorphous thin films are: initial vacuumless than 1×10⁻⁵ Torr; sputtering pressure of from 3×10² to 2×10⁻² Torr;pre-sputtering of a sputtering source of material to clear the surfacethereof; substrate temperature of 30° to 100° C., and an argon partialpressure.

In the cathodic sputtering process, argon gas ions bombard the solidalloy target cathode in the sputtering chamber dislodging metal atoms bytransferring the momentum of the accelerated ions to the metal atomsnear the surface of the target. The cathode is said to glow, and themass of ionized gas between the cathode and the anode is a plasma. Thesubstrate is placed at the anode, and the metal alloy atoms traverse thespace between the anode and cathode to deposit or condense on thesubstrate.

The object of this invention is to enable the manufacture of a magnetooptic medium which has a carrier-to-noise ratio of at least 45 decibelsand has an excellent quality of interaction between the medium and theoptical system to take advantage of the inherent quality of the medium.This involves not only maximizing theta, but also decreasing theintensity of the inherent noise level in the medium.

DISCLOSURE OF INVENTION

The present invention provides a new erasable magneto optical recordingmedium comprising a magnetizable, amorphous film, said film having amagnetic anisotropy perpendicular to the film surface, and said filmbeing characterized by having a multiplicity of magnetic domainssubstantially all of which have a domain size of less than 500 angstromsin largest dimension. Domain size as used herein means the greatestdimension of the domain measured in the plane of the film. These filmscomprise a material composition containing an element with an unpairedelectron, typically an alloy of at least one rare earth element and atleast one transition metal. This magnetizable amorphous film enables themanufacture of media having a characteristic carrier-to-noise ratio ofat least 47 decibels.

Although the magnetizable film is amorphous, it has different phases,which are defined as localized variations in density and/or compositionwithin the film. The existence of different phases adjacent to oneanother is believed to give rise to perpendicular anisotropy. Thisproperty makes it possible to magnetize a bit in the direction oppositethat of the film adjacent to it.

The characteristic magneto-optic angle of rotation of the magneto opticfilm, theta, is at least 0.24 degrees when measured with a helium neonlaser at a wave length of 6328 angstroms and at least 0.4° measured witha laser diode at a wavelength of about 7800 to 8500 angstroms.

Many film substrates can be used. They may be formed of any materialwhich is dimensionally stable, minimizing radial displacement variationsduring recording and playback. Semiconductors, insultors, or metals canbe used. Suitable substrates include glass, spinel, quartz, sapphire,aluminum oxide, metals such as aluminum and copper, and polymers such aspolymethyl-methacrylate and polyester. The substrate is typically in theform of a disc.

This medium can be incorporated into a multi-layer construction, whereinit is sandwiched between optical inteference layers. The resultingobserved effective magneto optic rotations of polarized light with thesandwich construction (in the range of 1 to 10° ) are relatively largeand represent an improvement over values of theta reported for rareearth transition metal (RE-TM) multi-layer constructions in theliterature.

The above-described erasable optical recording medium also has moreprecise bits (i.e. less average bit roughness) than known media, due tothe smaller size of the domains (usually about 100 angstroms). Bits aretypically 1 to 5 microns in longest dimension.

Although a Kaufman source or duoplasmatron could be used, the triodesputtering process is preferred for depositing the inventive amorphousthin films. Under ordinary, higher pressure cathodic sputteringconditions, the sputtered atoms lose kinetic energy through collisionwith gas molecules. The deposition rate is inversely proportional tosputtering pressure and the distance between the receiving surface andthe cathode. Triode sputtering, in addition to the main anode andcathode, has a thermionic cathode (emitter) and anode which has theadvantage that a plasma can be maintained at much lower pressures than adirect current glow discharge (even in a magnetic field or magnetron).The ability of a triode sputtering apparatus to maintain an argon plasmaat very low vacuums permits the deposition of these thin films atvacuums in the range of 4×10⁻³ to 6×10⁻⁴ Torr. The metal atoms whichdiffuse across the space between the sputtering cathode and anode areable to strike the substrate at a higher energy than they would at alower vacuum since there are fewer argon ions in the space to interferewith the motion of the metal atoms, giving a greater mean-free path.

With triode sputtering, there are also more nucleation sites on thesubstrate because of the statistically higher rate of bombardment byenergetic film constituent (metal alloy) atoms. This is believed to leadto a magneto optic film having a smoother surface than otherwise wouldoccur. This in turn, leads to a magneto optical recording medium inwhich the surface of the film yields a background noise level that is atleast 50 decibels below the carrier level when a 2 milliwattpeak-to-peak laser beam modulated at about 5.0 megahertz is reflectedfrom the unwritten medium moving at a linear velocity of about tenmeters per second. For the media of this invention, background noise isusually at least 65 decibels below the carrier level under theconditions stated above.

Although the media of this invention are erasable, they may be used inthe same application as write-once or non-erasable media.

BRIEF DESCRIPTION OF THE FIGURES

FIG. No. 1 is a transmission electron microscope photomicrograph at200,000 × of an amorphous metal alloy thin film magneto optical mediumof this invention.

FIG. No. 2 is an electron beam diffraction pattern of an amorphous metalalloy magneto optical thin film medium of the invention.

FIG. No. 3 is a profile of the electron beam diffraction pattern for theinventive magneto optic medium in FIG. 2.

FIG. No. 4 is a set of magnetic hysteresis loops for magneto optic filmsof this invention.

FIG. No. 5 is a set of optical hysteresis loops for magneto optic filmsof this invention.

DETAILED DESCRIPTION OF THE INVENTION

The good performance characteristics of these media (high θ and CNR) arethought to be attributable to physically identifiable features in themedia. The two features believed to be most important are the existenceof small domains and the optical constants of the recording medium (highrefractive index and low extinction coefficient). Index of refraction(n) and extinction coefficient (k) for a gadolinium-terbium-iron alloyfilm of this invention have been determined to be 4.5 and 1.8respectively. Domain size is preferably less than 200 angstroms inlargest dimension. Thus, a one micrometer bit can be made up of manymagnetized domains. FIG. 1 indicates domain boundary walls of 200angstroms or less in largest dimension.

Domain formation in magnetic materials is well known. However, theteachings of the prior art lead to the conclusion that as domain sizedecreases into the extremely small range of this invention, the domainswould be unstable. A bit comprised of unstable domains will generallyundergo observable changes within about two minutes after it has beenrecorded, such as changes in location on the medium, CNR and bit size. Aloss in CNR can indicate increased bit edge roughness. Stable bits areneeded in order to maintain the integrity of recorded data for longperiods.

A 51/4 inch (133 mm) diameter disc medium having a Gd-Tb-Fe amorphousalloy film of this invention coated thereon was tested for stability byrecording a series of bits at 9 milliwatts laser power using a biasmagnetic field of about 250 Oersteds (Oe). The recorded bits were readat 3.0 milliwatts laser power immediately after recording and about 14days later. Within the limits of experimental error there were nochanges in CNR, bit size or read signal amplitude between the tworeadings, indicating good bit and domain stability. Recorded bits on theinventive media have been stable for months.

The magnetizable amorphous film of the media herein described containsan element having an unpaired electron. One amorphous alloy compositionparticularly well-suited to this invention is gadolinium-terbium-ironternary alloy. The composition range preferred is 6-15 atom percentgadolinium, 10-20 percent terbium and 65-84 percent iron. Onecomposition made in the course of this invention was about 14% Gd, 17%Tb and 69% Fe. The Curie point temperature of this medium is about 120°C. The thin films made of this composition are generally greater than 50angstroms thick and have a coercivity sufficient to create a stablememory. At a minimum, this should be about 500 Oe, but a range of 2000to 3000 Oe is generally used.

As shown by FIG. 2, which is an electron beam diffraction pattern ofsuch a ternary alloy made on a 200 keV apparatus, these materials show adiffraction pattern with broad halos that are not easily assigned acrystalline structure. In diffraction patterns, amorphous character isindicated by line broadening to the point where individual linesoverlap. A broadened ring or fuzzy area which is divided by a concentricline is known as a split ring and indicates some short range ordering inthe amorphous character of the film.

The information from the photographic image of the diffraction patterncan be translated to a profile plot of the diffracted intensity versusdistance from the center of the diffraction pattern to give a moreprecise indication of amorphous character. The profiles of amorphousmaterials lack distinct peaks; whereas, those of crystalline materialshave a number of quite distinct peaks indicating the lattice spacing orspacing between the atomic orbitals within the lattice.

As explained before, a triode sputtering process is suitable fordepositing the magneto optic films of this invention. In the experimentsby which this process was reduced to practice, the argon used for thesputtering was ultrahigh purity, (99.999 percent minimum purity). Argonflow rate into the triode sputtering apparatus was about 50 standardcubic centimeters per minute (scc/min), at a pressure of about 1.3milli-Torr (which implies about 3 parts per million of gas present inthe system). This represents a decrease in the presence of oxygenpresent in and flushed through the system by a multiple of at least 20to 100 times less than ordinary direct current or radio frequencycathode sputtering.

The triode sputtering apparatus comprises a vacuum chamber containing asputtering cathode target where the metal alloy is placed. The alloysputters to provide an accumulation on the substrate which is placed onthe anode substrate holder. The anode is held at a low negative biasvoltage with respect to the chamber wall. The cathode target is watercooled, and the substrate can be made to rotate through an externaldrive means. A shutter is usually provided between the target and theanode to allow sputter cleaning of the substrate. Magnetically assistedtriode sputtering is preferred, in which a magnetic field is imposed inline with the thermionic cathode and anode to confine the electrons tothe plasma of ionizing gas and keep them away from the substrate whereelectron bombardment would cause heating. The sputtering chamber itselfis made of stainless steel.

The optical properties of an amorphous thin film are a function of boththe composition and the process by which the composition is formed ordeposited. It is known that rare earth metals oxidize readily, and thecontrol over this oxidation is an important part of the process of thisinvention to lead to a product of higher purity. If the anode is given anegative potential, with respect to the plasma, the resulting techniqueis referred to as bias sputtering. This bias is believed to cause apreferential removal of impurities such as oxygen from the main film byresputtering.

Radio frequency (RF) sputtering (rather than direct current) can be usedto effect cleaning and to deposit insulators, such as transparentdielectric films. In this technique, a radio frequency alternatingvoltage is applied to the sputtering chamber by means of RF electrodes.

In operation, the sputtering chamber is typically pumped down to someinitial background pressure (e.g. 4.0×10⁻⁷ Torr) after which the sputtergas (argon) is introduced. Typically, the substrate is cleaned bypre-sputtering or sputter etching for about 60 seconds at a bias voltageof about 300 volts. The substrate is exposed to the flux of atoms fromthe target after the predetermined sputtering conditions have beenreached. The deposition rate of the magneto optic film is generally 0.5to 4.0 angstroms per second in the case of the gadolinium terbium ironternary alloy. A thin film thermocouple is located near the anodesubstrate holder to measure the approximate substrate and equilibriumplasma temperature.

The higher vacuum of the triode apparatus appears to result in thinfilms of higher density and higher index of refraction than knownmagneto optic films such as those of U.S. Pat. No. 3,965,463.

It has been observed that the character of the magneto optic film at itssurface can be different from the bulk properties of the film. This hasbeen particularly evident in comparing coercivity measurements for thesurface and the bulk of an unpassivated film. H_(c) (coercivity) hasbeen found to vary by an order of magnitude in extreme cases. Thesechanges are especially important in an optical memory system, since theinteraction of the read optical beam and the RE-TM storage materialsoccurs in the first 150 to 200 angstroms of the film. Oxidation of therare earth is suspected of being the main cause of changes in thecharacteristics of thin film at the surface.

Passivation is the change of a chemically active metal surface to a muchless reactive state. By coating the RE-TM films with a passivationlayer, typically consisting of less than 300 angstroms thick of SiO_(x),(x less than 2) the change in characteristics with time has been nearlyeliminated, and higher magneto optic rotations have been measured thanwere previously obtained for RE-TM films without such a layer. Thisrepresents a significant increase in the stability of rareearth-transition metal magneto optical memory materials. Other materialsuseful for the passivating layer are: titatanium dioxide, SiO₂, ceriumoxide, aluminum oxide, and aluminum nitride.

A depth profile of elements in a sample of the inventive media having aGd-Tb-Fe alloy film passivated by a covering of SiO_(x) glass was madeby Auger Electron Spectroscopy (AER) and by secondary ion massspectroscopy (SIMS). The results indicated oxygen level in the Gd-Tb-Fefilm of less than one atom percent. Electron Spectroscopy for ChemicalAnalysis (ESCA) has shown the SiO_(x) films deposited over the Gd-Tb-Fefilms to have of 1.2-1.6 or an oxygen content of 55-62 atom percent.Depth profile analysis shows oxygen level within the Gd-Tb-Fe films tobe about 200 times less than it is in the SiO_(x), or by implicationabout 0.3 atom percent.

This invention will be further clarified by considering the exampleswhich will follow in this description. They are intended to be purelyexemplary.

EXAMPLE I

A magneto optic film of a specified thickness was deposited on areflector. This bi-layer was then overcoated with a third layer ofSiO_(x) dielectric. The choice of the reflector generally relates to itsefficiency of reflectivity at the wavelength of interest. The thicknessof the magneto optic film will be dependent upon its optical propertiesas observed at the wavelength of the light of interest. The magnetooptic material must be semi-transmissive in order to obtain increases inrotation from both the Faraday and Kerr effects. Films of Gd (11 atompercent) Tb (11 atom percent) with the balance being Fe (as determinedby x-ray fluorescence) were deposited on copper coated (reflectivelayer) and uncoated plain glass slides. The angle of rotation, theta,was measured both with a helium neon (HeNe) laser, (wavelength 6328angstroms) and a laser diode (L. D. wavelength 8300 angstroms). Theresults are given in Table 1 below.

                                      TABLE 1                                     __________________________________________________________________________    Substrate                                                                     Sample                                                                              Cu Coated                                                                           Uncoated                                                                           Magneto-Optic                                                                         SiOx  Rotation Angle                                 Number                                                                              Glass Glass                                                                              Film Thickness                                                                        Thickness                                                                           HeNe                                                                              L.D.                                       __________________________________________________________________________    132   X          385 Å                                                                             270   0.27                                                                              0.47                                       139   X          385 Å                                                                             950   1.12                                                                              1.10                                       138   X          385 Å                                                                             1025  1.32                                                                              1.37                                       140   X          385 Å                                                                             1065  1.27                                                                              1.55                                       145 on 144                                                                          X          385 Å                                                                             1100   .45                                                                              1.71                                       141   X          385 Å                                                                             1200  1.21                                                                              1.69                                       142   X          385 Å                                                                             1240  0.87                                                                              1.96                                       143   X          385 Å                                                                             1300  0.45                                                                              1.73                                       130   X          300 Å                                                                             300   0.36                                                                              0.47                                       129   X          300 Å                                                                             435   0.35                                                                              0.44                                       128   X          300 Å                                                                             610       0.70                                       126   X          300 Å                                                                             740       0.85                                       133   X          300 Å                                                                             878   1.75                                                                              1.21                                       137   X          300 Å                                                                             950   1.30                                                                              1.43                                       134   X          300 Å                                                                             1010  0.86                                                                              1.68                                       136   X          300 Å                                                                             1050  0.99                                                                              1.55                                       135   X          300 Å                                                                             1195  0.68                                                                              1.48                                       132         X    385 Å                                                                             270   0.08                                                                              0.17                                       139         X    385 Å                                                                             950   0.18                                                                              0.23                                       138         X    385 Å                                                                             1025  0.41                                                                              0.29                                       140         X    385 Å                                                                             1065  0.36                                                                              0.25                                       141         X    385 Å                                                                             1200  0.24                                                                              0.18                                       142         X    385 Å                                                                             1240  0.58                                                                              0.26                                       143         X    385 Å                                                                             1300  0.47                                                                              0.54                                       130         X    300 Å                                                                             300   0.04                                                                              0.11                                       129         X    300 Å                                                                             435   0.06                                                                              0.17                                       128         X    300 Å                                                                             610                                                  126         X    300 Å                                                                             740   0.01                                                                              0.11                                       133         X    300 Å                                                                             878   0.03                                                                              0.12                                       137         X    300 Å                                                                             950   0.31                                                                              0.12                                       134         X    300 Å                                                                             1010  0.67                                                                              0.15                                       136         X    300 Å                                                                             1050  0.19                                                                              0.16                                       135         X    300 Å                                                                             1195  0.73                                                                              0.14                                       144   X          385 Å                                                                              0    0.23                                                                              0.40                                       147   X          970 Å                                                                             950   0.70                                                                              0.76                                       147         X    970 Å                                                                             950   0.68                                                                              0.75                                       __________________________________________________________________________

The thicknesses of the magneto optic film and the silicon suboxide filmwere measured by a double beam interferometer. These results indicatethe dependence of rotation angle on both the thickness of the magnetooptic film and that of the interference film. They also indicate theclear advantage of using a magneto optic film deposited over areflector.

Various apparatus and methods are available and known to the art forreading data stored in magneto optic media, as is illustrated in U.S.Pat. No. 3,651,281 FIG. 3 and column 6 and 7. The same sort ofarrangement may be used to test magneto optic media with somemodifications. The basic testing apparatus comprises a laser diodeemitting a beam which is directed by means of various dielectricmirrors, plano mirrors, polarizers, lenses and prisms to the magnetooptic medium, and from there to a set of detectors which translate theintensity of the rotated light beam into an electronic signal. The laserdiode emits a diverging beam at a wavelength about 8300 angstroms, whichis polarized, modulated, collected and collimated by a lens and madecircular by a prism. This circular beam is directed by mirrors through afocusing head onto the medium. By virtue of the combined Kerr andFaraday rotations, the plane of polarization of the light is rotatedthorugh angle theta. Upon reflection, all of the rotated light, as wellas part of the non-rotated light, is directed onto a read path throughpolarizing beam splitters and toward photodiode detectors. Focusing ofthe read beam onto the medium can be done by imaging optics means (e.g,a TAOHS type 0.6 NA lens head from Olympus Co.)

A biasing magnet is located near the medium at the point where the beamstrikes it. The bias field used in recording can influencecarrier-to-noise ratio. However, as long as the bias field is more thanabout 300 Oersteds CNR is relatively independent of bias field strength.

The triode sputtering process can be used to control several parametersof the final magneto optic film. The magnetic and optical hysteresiscurves depicted in FIGS. 4 and 5 represent the characteristics of threegadolinium, terbium, iron alloy films produced by triode sputteringunder 3 different conditions. The films were all of the same composition(13.5 percent gadolinium, 16 percent terbium and 70 percent iron), andthey were all about 2000 angstroms thick. Film DC1 was sputtered on at130° C.; DC2 at 200° C.; and DC11 at 270° C. It is the more rectangularhysteresis loops of Sample DC11 which are desired.

An additional series of experiments showed that a decreasing rare earthconcentration results in an increased tendency to form a longitudinalcomponent in the magneto optic film. An applied magnetic field aroundthe target in the sputtering process also results in the formation of alongitudinally oriented medium. On the other hand, an increase in thedeposition rate results in the suppression of the longitudinal componentof the ternary alloy to some extent.

The different amorphous phases, which are necessary to the existence ofdomains adjacent to each other, appear to be dependent upon depositionrate, temperature, and film thickness.

Also, the anode or substrate bias has been found to be a useful controlin the triode sputtering process. In one series of experiments, keepingjall conditions the same except for substrate bias, which was variedfrom 0 volts to 623 volts, it was found that the rectilinearity of themagnetic hysteresis curve was increased substantially with increasinganode bias.

When a magnetizable amorphous film is deposited on a reflector, it isknown that the magneto optic rotation is increased because the Faradayeffect is added to the Kerr effect. The former effect rotates the planeof polarization of the light as it passes back and forth through themagneto-optic layer while the Kerr effect rotates it at the surface ofthe layer. Therefore, it is beneficial to deposit the magnetizableamorphous films on a substrate which has been reflectorized. Typicalreflecting layers are copper, aluminum or gold.

The performance of these media may also be improved by interferenceenhancement. The read beam reflected from a magneto optic medium has aregular component I_(x) and a magneto optically induced component I_(y).The magnitude of the light rotation effect I_(y) is determined both bythe inherent properties of the medium and the ability to transmit therotated radiation from the medium to some outside detecting means. Thislatter aspect is affected by optical interference layers. In addition tooptical effects, an overlayer of transparent dielectric, such as glass,can reduce the effect of oxidation on the amorphous metal alloy. Suchoverlayers also reduce the effect of dust and impurities on thetransmission of the read beam. These media are generally known asinterference enhanced media. The recording sensitivity of the magneticthin layer varies in response to the thickness of the dielectricinterference layer covering it and also with the magneto optic amorphousalloy composition and the wave length of the incident light.

The dielectric anti-reflection coating on top produces some gain;however, further increases in the efficiency (of conversion can bebrought about by a tri-layer film in) which the magneto optic film isdeposited on a transparent dielectric film which itself rests on anopaque reflector. This intermediate dielectric layer ideally has athickness of some integer multiple of λ/4, where λ is the wavelength ofthe read beam light. When this condition is met, the rotated component(I_(y)) of the magneto optically induced light, which is emitted bothforwards and backwards from the magneto optic thin film, is maximized byin phase addition of the exiting rays. An overlayer of a (transparentanti-reflective dielectric film can also be) used in conjunction withthe intermediate layer. In this case, the structure is known as aquadrilayer structure.

EXAMPLE II

A polymerically subbed polished aluminum disc, having a diameter of 30centimeters was provided. This disc had been made by coating a polishedaluminum disc, which had been previously cleaned, with a polymer (e.g.styrene-butadiene polymer). A solution of the polymer (e.g. about 4%solids in a solvent with a boiling point greater than about 140° C.),had been applied to the disc (while it was spinning). The solvent wasevaporated, leaving a thin polymeric subbing layer. The function of thesubbing layer is to provide a very smooth surface for recording. Thepolymer should wet and adhere to the aluminum surface.

The subbed disc was coated with a priming layer of chromium oxide (topromote adhesion of the reflecting layer to the substrate) by magnetronsputtering using a chromium target in an atmosphere of argon, watervapor and air. The chromium oxide sputtering was continued for about oneto two minutes at a target current of about 500 mA and a backgroundoperating pressure of about 2×10⁻⁵ Torr, thereby obtaining a nucleation,adhesion-promoting layer of about 40 angstroms thick. Other suitablepriming materials would be the oxides of titanium, tantalum andaluminum.

Over this, a reflecting copper layer about 1000 angstroms thick wasapplied by vacuum, resistance evaporation at a background pressure ofabout 2×10⁻⁶ Torr. The substrate thus prepared was cleaned by sputteretching for about 60 seconds at a bias voltage of about 300 volts in thepresence of argon. An intermediate glass film of silicon suboxide(SiO_(x)) was deposited from a silicon monoxide smoke source (obtainedfrom RD Mathis Company in Long Beach, Calif. to a thickness of about 250angstroms by sputtering.

The triode sputtering method was then used to coat the preparedsubstrate with gadolinium, terbium, iron alloy. High-purity argon gaswas leaked into the triode sputtering apparatus creating a backgroundpressure of about 1.2×10⁻³ Torr, and the deposition of the ternary alloyfilm was carried out under a substrate bias of about 300 volts and atarget bias of about 300 volts. The deposition rate was in the range of2.5 to 3 angstroms per second with a final film thickness of about 285angstroms. At a vacuum of less than about 9.0×10⁻⁷ Torr, a glassovercoat of about 1360 angstroms thick was deposited from an SiO_(x)smoke source.

The alloy target used to create this magneto optic film was a mosaic ofthe desired constituents. Final composition of the deposited films wasdetermined by energy dispersive x-ray fluoresence spectroscopy. Thecomposition of the produced sample number 34-195 was determined to be6.5 atom percent gadolinium, 10.0 percent terbium and 83.5 percent iron.

Table 2 indicates various magneto optic properties of Sample 34-195 andcompares them to certain published values of known magneto optic media.All the data for sample 34-195 was recorded and read at a 115 mm radiuson the disc.

                                      TABLE 2                                     __________________________________________________________________________    Sample     34-195                                                                            34-195                                                                            34-195                                                                            34-195                                                                            34-195                                                                            34-195                                                                            34-195                                                                            Pub. 1                                                                             Pub. 2                                                                             Pub. 3                                                                             Pub.                                                                               Pub.               __________________________________________________________________________                                                               5                  Write laser                                                                              6.0 7.0 8.0 9.0 10.0                                                                              11.0                                                                              12.0                                                                              8    6    6    5                       Power (mW)                                                                    Recording Fre-                                                                           2   2   2   2   2   2   2                       up to              quency (MHz)                                               2.5                Magnetic bias                                                                            600 600 600 600 600 600 600                                        field (Oe)                                                                    Resolution Band                                                                          30  30  30  30  30  30  30  30   30   30   30   30                 Width Frequency                                                               (KHZ)                                                                         Read Laser Power                                                                         2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.5  2.5  2.5  2.5  2                  (mW)                                                                          Carrier to Noise                                                                         43.4                                                                              49.2                                                                              50.5                                                                              51.5                                                                              52.2                                                                              52.4                                                                              52.7                                                                              40   40   44   40   40                 Ratio (dB)                                                                    Compensation                                                                             118 118 118 118 118 118 118 160  140  140  140                     Point (°C.)                                                            Disc speed (rpm)                                                                         780 780 780 780 780 780 780 1350 1350 1350 1350 720                __________________________________________________________________________

Samples Pub 1-4 are taken from Imamura, Nobutake, "The Development ofMagneto-Optical Disc Memory With Semi-conductor Lasers", KDD Researchand Development Laboratory, Tokyo, Japan, and the data for sample Pub-5is taken from Bell, Alan E., "Optical Data Storage" Laser Focus,Jannuary 1983.

FIG. 1 is a transmission electron microscope photomicrograph of themedium of sample 34-195 at 200,000 X. The very small domain size issuggested by this photograph. The space between the hash marks A denotesa distance of 200 angstroms, and the small identifiable spots, believedto represent domains, appear smaller than distance A in largestdimension.

FIGS. 2 and 3 are the electron beam diffraction pattern and profilerespectively for the medium of sample 34-195, showing its amorphouscharacter.

Except for the carrier to noise ratio at the lowest write laser power of6 milliwatts, the significantly greater than that of the publishedmedia. It significantly greater than that of the published media. It isfelt that a CNR of at least 60 can be obtained using the techniques andmaterials described above.

EXAMPLE III

A sputtering target was made by placing terbium chips onto an irontarget having an approximate area of 7742 mm². The chips were about10mm×25 mm in size, and the areal ratio used was 25.8% Tb to 74.2% Fe.

51/4 inch (133 mm) diameter polymethyl methacrylate (PMMA) discs wereused as substrates. The discs were grooved and had a subbing layer madeof 100% solids photopolymer cured with ultraviolet light. One substrateplus four slides (two PMMA and two glass) were loaded into thesputtering chamber, the disc being mounted on a rotating platen.

After pumping down the pressure to about 5.6×10⁻⁷ mbar, SiO_(x) wasevaporated by resistance heating from a baffled source filled withsilicon monoxide granules. It was deposited on the substrate at anaverage rate of about 5.5Å/sec. to a thickness of about 400Å. Pressureduring SiO_(x) evaporation was about 6.2×10⁻⁷ mbar, and afterevaporation it was about 7.3×10⁻⁷ mbar.

The next step was to radio frequency sputter etch the SiO_(x) justdeposited. This was done in argon using 80v for 30 seconds.

The triode sputtering apparatus was made ready for Tb-Fe sputtering. Theargon flow was set at 28.6 sccm (standard cubic centimeters per second)while the triode emitter was warming up. The triode was stable withargon pressure at 1.3×10⁻³ mbar. The direct current (d.c.) bias powersupply was turned on and warmed up to a constant voltage of 300 v and acurrent of 0.69 amps. The target was shuttered during these operationsto prevent premature deposition. The triode operated in this warmed upstate for about 30 seconds. The radio frequency substrate substrate biaswas turned on and adjusted to 80 v with 30 seconds duration.

At this point, the shutter covering the Tb-Fe target was opened and theradio frequency substrate bias adjusted to 200 v. The triode plasmasupply operated at 5 amps and 49 v; target bias at a constant 0.69 ampsat 300 v. d.c. The sputtering chamber pressure during sputtering was1.3×10⁻³ mbar. Average deposition rate was 1.5-2.0 Å/sec, and depositionof the Tb-Fe was terminated when the Tb-Fe film thickness was about275Å. Chamber pressure after this termination (gas flow off) was5.2×10⁻⁷ mbar.

SiO_(x) was then coated over the Tb-Fe layer by evaporation, asdescribed previously, to a thickness of about 290Å at a pressure of4.6×10⁻⁷ mbar. After a cool down time of about 30 minutes, the systemwas vented to dry nitrogen, opened up and the samples removed.

The disc and two of the slides (one plastic, one glass) were mountedonto a rotating substrate holder and placed in another vacuum chamber.After pumping down to a background pressure of 7.4×10⁻⁷ Torr, CrO_(x)primer was deposited over the second SiO_(x) layer. Oxygen at a flow of0.5 sccm and argon at 56 sccm were let into the vacuum chamber. Theargon and oxygen pressure with the throttle valve closed was 3.3×10⁻³.Deposition of CrO_(x) by magnetically assisted diode sputtering from achromium target proceeded for four seconds, giving a primer filmthickness of about 100Å.

Next a copper layer was deposited by resistance heating copper in amolybdenum boat. Background pressure was 8×10⁻⁷ Torr. Copper wasevaporated and deposited on the CrO_(x) primer at an average rate of 40Å/sec. up to a film thickness of about 1000Å. Background pressure aftercopper deposition was 10⁻⁶ Torr.

A final SiO_(x) layer was deposited over the copper layer by electronbeam evaporation of silica granules. Background pressure was 8×10⁻⁷Torr. The SiO_(x) was evaporated with the electron gun at 8 Kv and wasdeposited at a rate of about 15Å/sec. to a total thickness of about1200Å.

The disk and slides were removed from the vacuum chamber, and analysisshowed the magneto optic layer composition to be about 25 atom % Tb and75% Fe. The following data were measured on the disc:

    ______________________________________                                        specular reflectance at 820 nm                                                                       20.4%                                                  wavelength (using a spectro-                                                  radiometer)                                                                   Hc at room temperature 2900 Oe                                                Dynamic tests                                                                 CNR at write 1aser power of 9 mW                                                                     52                                                     background noise level 70 dB below                                                                   carrier level                                          ______________________________________                                    

EXAMPLE IV

The sputtering target as used in Example III was modified by placingeight cobalt chips, each being approximately 50 mm² in area, on the irontarget surface between the previously described Tb chips. The arealratios were 25.8% Tb, 67.2% Fe and 7% Co. The process was performed in asimilar manner to Example III with the following exceptions:

In depositing the first SiO_(x) film the sputtering apparatus was pumpeddown to 2.6×10⁻⁶ mbar, and the SiO_(x) film was deposited at about5.4Å/sec. to a thickness of about 800Å. Pressure during evaporation wasabout 2.2×10 hu -6 mbar after evaporation.

In sputtering the Tb-Fe-Co layer: the d.c. target bias was warmed up toa constant current of 0.63 amps, triode plasma supply ran at 47 v;target bias was at 0.63 amps; and chamber pressure after termination ofthe Tb-Fe-Co film deposition was 2.1×10⁻⁶ mbar.

The second SiO_(x) coating step was at a pressure of 2.0×10⁻⁶ mbar anddeposited SiO_(x) at 5.1Å/sec.

The vacuum chamber was pumped down to a background pressure of 9×10⁻⁷Torr prior to CrOx deposition.

For the copper coating step, background pressure was 9×10⁻⁷ Torr, andchamber pressure after deposition was 1.24×10⁻⁶ Torr.

Background pressure for the deposition of the final SiO_(x) layer was9×10-7 Torr.

Analysis showed the magneto-optic layer to have the followingcomposition: 23% Tb, 66% Fe and 11% Co.

The following data were measure on the disc:

specular reflectance at 820 nm wavelength - 14.1%

room temperature Hc - 2600 Oe

Dynamic tests

CNR at write laser power of 12 mW - about 53 background noise level -67.4 dB below carrier level.

While certain representative embodiments and details have been shown forthe purpose of illustrating the invention, it will be apparent to thoseskilled in this art that various changes and modifications may be madein this invention without departing from its true spirit or scope whichis indicated by the following claims.

I claim:
 1. A magneto-optical recording medium comprising a substrateand a passivated magnetizable amorphous film comprising a rareearth-transition metal alloy having a magnetic anisotropy perpendicularto the film surface, an oxygen concentration greater than or equal to0.3 atom percent and yet less than one atom percent, said magneto-opticrecording medium having a rotation angle for polarized light reflectedfrom the recording medium of at least 0.4° measured with a laser diodeat a wavelength of about 7800 to 8500 angstroms.
 2. The magneto-opticalrecording medium of claim 1 which further comprises a reflective surfaceon one side of the magnetizable amorphous film.
 3. The magneto-opticalrecording medium of claim 2 wherein the rotation angle is measuredwithout any additional layers on the recording medium, which wouldincrease rotation angle, besides the substrate, the magnetizableamorphous film and the reflective surface.
 4. A magneto-opticalrecording medium comprising a substrate, a passivated magnetizableamorphous film comprising a rare earth-transition metal alloy having amagnetic anisotropy perpendicular to the film surface, and a reflectivesurface on one side of the magnetizable amorphous film, wherein saidmagneto-optical recording medium has:(a) an oxygen concentration in themagnetizable amorphous film of greater than or equal to 0.3 atom percentand yet less than 1 atom percent; and (b) a rotation angle for polarizedlight reflected from the recording medium of at least 0.4° measured witha laser diode at a wavelength of about 7800 to 8500 Angstroms, saidmeasurement being made without any additional layers, such as opticalinterference layers which would increase rotation angle, on therecording medium besides the substrate, the magnetizable amorphous filmand the reflective surface.